174 PHOLADID^E. 



having I trust been satisfactorily disposed of, and every other 

 part of the animal exhibiting a prototype similarity, it is 

 impossible, as I think, to entertain further doubts of the 

 positive identity of the two shells usually termed by authors 

 Pholas papyracea and P. lamellata. I may add, that it has 

 been asked in objection, how is it that twenty Pholadidea 

 papyracea are taken for one Pholas lamellata, and that the 

 two forms are not more frequently met with in the transition 

 states? This objection quickly yields to a just view of the 

 Pholades as regards habitat and other influences. 



The great mass of the Pholades inhabit the littoral and 

 laminarian zones, and are thus open to the attacks of a multi- 

 tude of enemies ; the delicate young papyracea suffer severely. 

 My dredger, who for more than forty years has annually dug 

 large quantities of all the species except P. crispata, states, 

 that he constantly finds the shells of the young papyracea in 

 their cavities, cleared out of the animal by minute species of 

 Crustacea, in such quantities, that the dealers set them 

 up with gum as a substitute for the rarer live examples, but 

 the domed shells are rarely met with ; still a portion escape 

 to maintain the race, and some few young occasionally 

 occur. I have personally taken them, in situ, in a decided 

 transition state; it is necessary to observe that the crafty 

 dealers produce this condition by mutilating the dome, but a 

 lens will show the artful fracture or the natural progress of 

 growth. 



I will say a few words on the pelagic Pholades inhabiting 

 masses of stone dredged up in the littoral zones of the Devon 

 coasts, six or eight miles from land. These shells, whether 

 they are the two forms of Pholadidea papyracea, or the Pholas 

 parva or P. dactylus, are always dwarf. I have a curious 

 series of minute and completely adult Pholadidea papyracea 

 not exceeding a of an inch in length. Such shells are 

 considered by the inexperienced observer as proofs that at all 

 ages the Pholadidea papyracea is completely covered with a 

 dome and continues gradually to increase : this is impossible, 

 as when the dome and caliciform posterior extremity are once 

 formed, all further growth is for ever terminated. The pelagic 



